video_attachment: Only preload metadata

This hints browsers that only the metadata and maybe the first few frames are
needed. Which avoids downloading the entire video, which isn't really needed
in our case.

src/components/video_attachment/video_attachment.vue

@@ -1,6 +1,7 @@
 <template>
   <video
     class="video"
+    preload="metadata"
     :src="attachment.url"
     :loop="loopVideo"
     :controls="controls"